Micron-scale ultrasound identification sensing tags

ABSTRACT

Apparatus and methods for powering micron-scale implantable and injectable integrated circuit (IC) chips for in-vivo sensing and acquisition of various physiological signals are provided. The disclosed subject matter includes the integration of piezoelectric transducers, such as polyvinylidene fluoride (PVDF) or lead zirconate titanate (PZT), onto implantable and injectable IC chips for power transfer and data transmission using ultrasound waves generated from commercial ultrasound imaging equipment.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of International Patent ApplicationNo.: PCT/US2016/050165 filed Sep. 2, 2016, which claims priority to U.S.Provisional Application Ser. No. 62/214,822, filed on Sep. 4, 2015, theentire contents of which is incorporated by reference herein.

BACKGROUND

Implantable and injectable electronic devices containing integratedcircuit (IC) chips can aid, monitor, or support the functions of organsand even cells, and can be utilized in diagnosis and treatment ofmedical conditions when implanted or injected into various organs orcells. The sizes of the implantable and injectable devices can vary as aresult of the inclusion of components designed to carry out the devicefunctionalities, including the power sources. Batteries can be used aspower supplies for implantable and injectable devices. However,batteries can often be bulkier than the IC chips of the implantable andinjectable devices, and can require periodic replacement and recharge,limiting the applicability of those devices. Unlike the battery-basedpower scheme, wireless power transfer from external energy-emittingsources to the implantable and injectable devices can reduce the devicesize by removing the battery component.

One technique for wirelessly delivering power across biological tissuesto implantable and injectable devices is that of inductive coupling. Forexample, inductive coupling uses a pair of antennas by which power canbe transferred across biological tissues via electromagnetic (EM) wavesgenerated from a mutual-inductively coupled link. While attractive whenthe distance between this pair of antennas is small, the antenna sizecan be larger than the sizes of the device IC chips. Additionally,certain tissue attenuation of EM waves and the drop of power transferefficiency when the distance between antennas increases can limit theamount of delivered power and therefore the performances of implantableor injectable devices powered by inductive coupling.

Accordingly, there is a need for improved devices and techniques forwirelessly powering implantable and injectable devices.

SUMMARY

The presently disclosed subject matter provides devices and techniqueswhich integrate piezoelectric transducers, such as polyvinylidenefluoride (PVDF) or lead zirconate titanate (PZT), with customized ICchips to form electronic devices that can be implanted or injected intoorgans/cells. These implantable and injectable devices can be wirelesslypowered by ultrasound waves and can wirelessly transmit data alsothrough ultrasound, therefore referred to as ultrasound identification(USID) sensing tags. The ultrasound waves can have a frequency from 1MHz-50 MHz, depending on the size of the tag, and can be generated fromcertain commercial ultrasound imaging equipment, such as, for exampleand without limitation, the Verasonics Vantage 256 system.

Ultrasound can provide improved power transfer efficiency throughbiological tissues and enables much smaller device size due to a muchsmaller wavelength compared to certain other wireless power transfertechniques. For example and without limitation, the wavelength of EMwaves at 40 MHz are a few meters in biological tissues, while thewavelength of ultrasound at 40 MHz is approximately 37.5 μm, which isorders of magnitude smaller than that of EM waves at the same frequency.

In an exemplary embodiment, the disclosed subject matter can includetens and even hundreds of micron-scale USID sensing tags implanted orinjected into organs/cells of interest. Each individual tag can includea customized IC chip with a piezoelectric transducer integrated on thetop surface of the chip. In some embodiments, the IC chips can be formedusing a conventional complementary metal-oxide-semiconductor (CMOS)process. The CMOS process can be either a silicon-on-insulator (SOI)CMOS process or a bulk CMOS process. Each individual IC chip can bedesigned to have a length by width of 150 μm by 150 μm (althoughdifferent sizes depending on the wavelength of the ultrasound can beused), and multiple IC chips can be contained in a single die. The diethickness can be approximately 300 μm in conventional CMOS processes.

Die/chip thinning processes with a sequential combination of mechanicalgrinding followed by deep reactive-ion etching (DRIE) can be utilized toreduce the die thickness to approximately 20 μm. The reduced diethickness not only largely decreases the overall device size of the USIDsensing tags for various implantation/injection applications, but alsoreduces the mechanical loading effects on the piezoelectricity of theintegrated piezoelectric transducer. Additionally, the integratedpiezoelectric transducers can have a length by width by thickness ofapproximately 150 μm by 95 μm by 28 μm. In such a way, the USID sensingtags can have a size of approximately 150 μm by 150 μm by 48 μm,therefore referred to as “micron-scale.”

When excited emitted energy, such as from, commercial ultrasound imagingequipment, an implanted or injected USID sensing tag can reflect part ofthe received ultrasound waves as echoes back to the equipment, which canform a brightness-mode (B-mode) ultrasound image showing the shape,location, and even movement of the tag. The level of acoustic impedancemismatch between the tag and the surrounding tissues can determine theamplitude of the returned echo signal, which then can determine thebrightness of the tag in the ultrasound image.

When receiving ultrasound waves from the ultrasound imaging equipment,the herein disclosed piezoelectric transducer of a USID sensing tag canconvert mechanical vibrations of ultrasound waves into electrical energyin the alternating-current (AC) form to provide power for the IC chip.Additionally, the herein disclosed IC chip of a USID sensing tag caninclude a front-end charge pump, a relaxation oscillator, and amodulator. The charge pump can convert the AC electrical energygenerated in the integrated piezoelectric transducer into adirect-current (DC) supply to power the relaxation oscillator. Therelaxation oscillator can generate a sub-10-Hz oscillation to drive themodulator.

The modulator can actively modulate the input impedance of the IC chipto cause a periodic impedance change seen by the piezoelectrictransducer. This periodic change in input impedance can cause a periodicchange in the acoustic impedance mismatch between the tag and thesurrounding tissues and therefore the reflectivity of ultrasound waves,or the amount of returned echo signal to form a B-mode image in theultrasound imaging equipment. This can cause a periodic brightnesschange of the USID sensing tag in the image and can indicate thepowering and detection of the tag. Additional circuitry can be added andconfigured for sensing functionality with transduced voltagesanalog-to-digital-converted with a voltage-controlled oscillator, suchthat, for example and without limitation, changes in voltage can producediscernible changes in oscillation frequency of the backscatteredultrasound energy. Exemplary functionalities can include real-time pHsensing, drug delivery and controlled release, chemical and biologicalactivity detection, and digestion monitoring of small animals.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts the operation of multiple USID sensing tags in theorgan/cell of interest in accordance with an exemplary embodiment of thedisclosed subject matter.

FIGS. 2A-2I depict a fabrication process for producing the USID sensingtags in accordance with an exemplary embodiment of the disclosed subjectmatter.

FIG. 3 depicts a circuit block diagram in accordance with an exemplaryembodiment of the disclosed subject matter.

FIG. 4 depicts a circuit schematic diagram in accordance with anexemplary embodiment of the disclosed subject matter.

FIG. 5 depicts a circuit schematic diagram in accordance with anexemplary embodiment of the disclosed subject matter.

FIG. 6 depicts a circuit schematic diagram in accordance with anexemplary embodiment of the disclosed subject matter.

FIG. 7 depicts a die diagram in accordance with an exemplary embodimentof the disclosed subject matter.

It is to be understood that both the foregoing general description andthe following detailed description are exemplary and are intended toprovide further explanation of the disclosed subject matter claimed.

The accompanying figures, where like reference numerals refer toidentical or functionally similar elements throughout separate views,serve to further illustrate various embodiments and to explain variousprinciples and advantages all in accordance with the disclosed subjectmatter. For purpose of explanation and illustration, and not limitation,exemplary aspects and embodiments of the device are shown in FIGS. 1-7.

DETAILED DESCRIPTION

The presently disclosed subject matter provides micron-scale implantableand injectable USID sensing tags, where certain IC chips are integratedwith piezoelectric transducers. The USID sensing tags can be wirelesslypowered and detected/imaged by ultrasound waves at 1-50 MHz from certaincommercial ultrasound imaging equipment. The IC chips can be implementedin conventional CMOS processes and can be fabricated in commercialfoundries. The USID sensing tags can be fabricated by integrating thepiezoelectric transducers with the IC chips using a series offabrication processes.

For the purpose of illustration and not limitation, reference is made tothe exemplary USID sensing tag 100 shown in exemplary FIG. 1. FIG. 1illustrates the general operation of the presently disclosed subjectmatter. For example, the fabricated USID sensing tag 100 can beimplanted or injected into a material of interest 120, such as an organ,a cell, soft tissue, or water. The scanhead 130 of commercial ultrasoundimaging equipment, such as for example, the Verasonics Vantage 256system, transmits ultrasound waves 131 to the USID sensing tag 100through the material of interest 120. Additionally or alternatively, andas shown in exemplary FIG. 1, an ultrasound coupling gel 135 can beapplied in between the scanhead 130 and the surface of the material ofinterest 120 to improve the transmission of ultrasound waves 131 fromthe scanhead 130 to the USID sensing tag 100. The ultrasound couplinggel 135 can, for example, improve transmission of ultrasound waves 131since ultrasound waves 131 can exhibit inferior transmission through airthan through the ultrasound coupling gel 135. The mechanical energy ofthe ultrasound waves 131 of exemplary FIG. 1 can be converted toelectrical energy by the piezoelectric transducer 101 integrated ontothe IC chip 102 of the USID sensing tag 100. The electrical energyconverted from ultrasound waves 131 can power the IC chip 102 forsensing and data transmission.

Further to the above, the disclosed subject matter can include a methodfor fabricating the USID sensing tags, which can include, for exampleand without limitation, thinning and surface planarization of the diethat contains the IC chips, separation of individual IC chips on thedie, passivation of the individual IC chips for biocompatibility,integration of the individual IC chips with their correspondingpiezoelectric transducers, and release of the resulting devices from thedie. For example, a die that contains the IC chips can be thinned from athickness of approximately 300 μm down to a thickness of approximately20 μm, which renders the originally brittle die thin and flexible.Exemplary techniques for thinning chip die can include a sequentialcombination of mechanical grinding followed by DRIE (deep reactive-ionetching). Other suitable thinning techniques are also within the scopeof the presently disclosed subject matter.

For example and without limitation, a method of fabricating the USIDsensing tags is illustrated in exemplary FIGS. 2A-2I. As shown inexemplary FIG. 2A, the thinned IC die 201 can be placed on top of abacking layer made of aluminum for mechanical support and thentransferred onto a carrier substrate for convenient handling during thesubsequent fabrication processes. The carrier substrates can be, forexample, materials capable of withstanding the subsequent fabricationprocesses, such as a semiconductor wafer or glass. Between exemplaryFIGS. 2A to 2B, the top surface 211 of the die 201 containing the ICchips is planarized by uniformly etching the passivation layer usingreactive-ion etching (RIE).

Next, as shown in exemplary FIG. 2C, the individual IC chips on the die201 are separated by performing dry etching in RIE for front-end-of-line(FEOL) 201 a, back-end-of-line (BEOL) 201 b, and buried oxide (BOX) 201c to form vertical trenches 202 around the active areas of the IC chips.For example and without limitation, the FEOL 201 a can refer to thefirst part of the IC chips that contain individual components, such astransistors, resistors, and capacitors. The BEOL 201 b can refer to thesecond part of IC chips where these individual components getinterconnected with metalization layers. The BOX 201 c can refer to aninsulator layer made of oxide and buried in the silicon wafer of SOICMOS processes. As shown in exemplary FIG. 2D, the sidewalls of the ICchips in their respective vertical trenches 202 can be passivated.Passivation of the sidewalls can include for example, depositingconformal parylene to fill the vertical trenches 202 thereto to permitbiocompatibility of the IC chips with biological tissues.

An anisotropic conductive adhesive can then be deposited onto thesurface 211 of the IC chips, as illustrated in exemplary FIG. 2E. Forexample and without limitation, the adhesion material selected anddeposited can conduct vertically only and not laterally in order toavoid shorting adjacent pads/electrodes. Following the deposition of theadhesion layer 203, the fabrication process of the presently disclosedsubject matter can include integrating piezoelectric transducers ontothe IC chips. Such integration is illustrated in exemplary FIG. 2F. Eachindividual IC chip contains an input pad and a ground pad, and apiezoelectric transducer has a top electrode and a bottom electrodebetween which the electrical energy converted from the incomingultrasound waves is produced. The integration of a piezoelectrictransducer with an IC chip can include the electrical connections of thebottom transducer electrode with the chip input pad and the toptransducer electrode with the chip ground pad. For example and withoutlimitation, a piece of piezoelectric transducer 204 that completelycovers the top surface 211 of the die 201 containing the IC chips can beplaced on top of the adhesion layer 203 to make the electricalconnection between the input pads of the IC chips and the correspondingbottom transducer electrodes.

As shown in exemplary FIG. 2G, the piezoelectric transducer 204 is thenpatterned with the AZ4620 photoresist as an etch mask and subsequentlyetched in RIE to expose the ground pads of the IC chips. Other suitablepatterning techniques are also within the scope of the presentlydisclosed subject matter. The top transducer electrodes can then beelectrically connected to the chip ground pads via deposition oftitanium metal for biocompatibility, followed by a lift-off process toremove unwanted metallization, as illustrated in exemplary FIG. 2H. Thefabricated devices 100 can be released from the carrier substrate byselectively etching away the backing layer, as illustrated in exemplaryFIG. 2I. The released devices can be transferred to deionized water forrinsing. Such fully fabricated USID sensing tags 100 can be either driedor can be implanted/injected into a sample of interest using a syringe.

As disclosed herein, and with reference to exemplary FIG. 3, the USIDsensing tag can include an IC chip configured in accordance with thedisclosure herein. For example and without limitation, the IC chipherein disclosed and illustrated in exemplary FIG. 3 can be implementedin a 180-nm CMOS technology provided by Taiwan SemiconductorManufacturing Company (TSMC). An exemplary overall circuit block diagram300 of the IC chip is depicted by way of example and explanation only,and not limitation. The customized IC chip can include a front-endcharge pump 320, coupled to receive the AC input signal 311 from theintegrated piezoelectric transducer 310 and is configured to produce aDC output. The IC chip further includes a relaxation oscillator 330,coupled to the charge pump 320. The relaxation oscillator 330 can beconfigured to receive power from the DC output generated in the chargepump 320 and produce a periodic oscillation signal. The IC chip furtherincludes a modulator 340, coupled to the relaxation oscillator 330. Themodulator can be configured to receive the oscillation signal from therelaxation oscillator and accordingly modulate the input impedance ofthe IC chip.

The front-end charge pump 320 can be configured to convert the input ACsignal into a DC voltage to power the relaxation oscillator 330 as shownin exemplary FIG. 3. A circuit diagram for the charge pump is providedin FIG. 4. When the AC input 411 from the piezoelectric transducer isnegative, the current I₁ 401 charges C₁ 402 to the peak value of theinput through D₁ 403; when the input is positive, in addition to theexisting voltage across C₁ 402, the current I₂ 405 charges C₂ 406 todouble the peak voltage of the input through D₂ 407, creating a voltagethat is twice the peak valve of the AC input 411 at the DC output 421.Additionally or alternatively, C₂ 406 can also be used as a storagecapacitor to store the energy accumulated from the input source.

A circuit diagram for the relaxation oscillator 330, with reference toexemplary FIG. 3, is provided in FIG. 5. The relaxation oscillator 330of the presently disclosed subject matter can include three inverters,U₁, U₂ and U₃, a capacitor C₁ and a resistor as illustrated by way ofexample in FIG. 5. Such circuitry can be configured to produce aspecific oscillation frequency depending on the value of the capacitorand resistor. Additionally, the resistor in the oscillator of exemplaryFIG. 5 can be implemented using the cathodes of two diodes D₁ and D₂connected in series. These diodes can provide a high resistance on theorder of hundreds of Giga Ohms (Ge), and therefore can produce anoscillation frequency of a few Hz. The measured oscillation frequency ofthe present oscillator is approximately 1.8 Hz and measured powerconsumption is 3.24 nW.

With reference to exemplary FIG. 3, a modulator 340 can be configured toactively modulate the input impedance of the IC chip. As illustrated inexemplary FIG. 6, the modulator 340 can include a single n-channelmetal-oxide-semiconductor field-effect transistor (MOSFET) Q₁. TheMOSFET includes at least three terminals, including a gate, a source,and a drain, where the gate is connected to and controlled by the outputof the relaxation oscillator 330, the source is connected to ground andthe drain is connected to the ultrasonic AC input 311. Suchconfiguration can cause the input impedance of the USID sensing tag toappear as solely the piezoelectric transducer 310 when the oscillatoroutput is in the low state and as a parallel combination of thepiezoelectric transducer 310 and the capacitance between the drain andsource of the MOSFET when the oscillator output is in the high state.The modulator 340 of exemplary FIG. 3 is thereby implementing a standardamplitude-shift keying (ASK) scheme to alter the echo reflected from thepiezoelectric transducer 310 during excitation. For example and withoutlimitation, the size of the MOSFET can be 2 μm/450 nm (length/width) sothat the impedance at the ultrasonic input 311 can be decreased by afactor of approximately three when the oscillator output changes fromthe low state to the high state, thereby modulating the reflectedultrasound waves back to the ultrasound imaging equipment. By way ofexample only, and not limitation, the B-mode ultrasound image scan canbe used to confirm the functionality of the presently disclosed subjectmatter. When a USID sensing tag is functional upon excitation, the tagappears as a blinking dot in the ultrasound image as its brightnessperiodically changes according to the modulation of the reflectedultrasound echo signals back to the imaging equipment.

Exemplary FIG. 7 shows a diagram of the die containing the IC chipsimplemented in a 180-nm CMOS technology provided by TSMC.

As herein disclosed, the commercial ultrasound imaging equipmentutilized to provide ultrasound waves to the USID sensing tags can bedesigned for medical imaging of small animals in a preclinical setting,including, but not limited to, cardiac, vascular, tumor and molecularimaging for mouse, rat, rabbit and zebra fish. Additionally, the sizeand power requirement of the micron-scale USID sensing tags matches thespatial resolution and energy intensity of the ultrasound waves in thechosen 30 MHz-50 MHz range from the imaging equipment. Also, the USIDsensing tags can potentially incorporate sensors for in-vivo acquisitionof various physiological signals. Therefore, the USID sensing tags ofthe presently disclosed subject matter can be implanted or injected intovarious organs in several kinds of small animals and excited/imaged bythe commercial ultrasound imaging equipment for numerous applicationsand studies. With different kinds of embedded sensors, applications ofthese tags include, but not limited to, monitoring electrophysiology inthe brain through the vasculature, probing intracellular activities byexamining concentrations of certain molecules, and detecting biogenicamine levels in the gastrointestinal tract to study communicationbetween the microbiota and the brain.

The description herein merely illustrates the principles of thedisclosed subject matter. Various modifications and alterations to thedescribed embodiments will be apparent to those skilled in the art inview of the teachings herein. Accordingly, the disclosure herein isintended to be illustrative, but not limiting, of the scope of thedisclosed subject matter. Moreover, the principles of the disclosedsubject matter can be implemented in various configurations of hardware,and are not intended to be limited in any way to the specificembodiments presented herein.

The invention claimed is:
 1. An ultrasound identification sensing tagpowered by ultrasound signals from commercial ultrasound imagingequipment, comprising: a piezoelectric transducer configured to convertmechanical energy of said ultrasound signals into alternating-current(AC) electrical energy; and an integrated circuit chip, electricallycoupled to said piezoelectric transducer, comprising: (i) circuitryconfigured to at least receive said electrical energy for power and toactively modulate an input impedance of the integrated circuit chip, and(ii) a relaxation oscillator, coupled to a charge pump, and configuredto receive a DC output voltage from said charge pump and generate anoscillation signal, and wherein said relaxation oscillator comprisesthree inverters, one capacitor, and two diodes, wherein said integratedcircuit chip and said piezoelectric transducer are integrated.
 2. Theultrasound identification sensing tag of claim 1, wherein saidultrasound signals from said commercial ultrasound imaging equipmentcomprise ultrasound waves within a range of 1 MHz to 50 MHz.
 3. Theultrasound identification sensing tag of claim 1, wherein saidintegrated circuit chip is passivated by parylene.
 4. The ultrasoundidentification tag of claim 1, wherein said piezoelectric transducerincludes at least one of polyvinylidene fluoride (PVDF) or leadzirconate titanate (PZT).
 5. The ultrasound identification sensing tagof claim 1, wherein said integrated circuit chip further comprises: thecharge pump, coupled to receive said AC electrical energy from saidpiezoelectric transducer, configured to produce a direct-current (DC)output voltage; a modulator, coupled to said relaxation oscillator,configured to receive said oscillation signal from said relaxationoscillator and modulate said input impedance of said integrated circuitchip.
 6. The ultrasound identification sensing tag of claim 5, whereinsaid charge pump further comprises two capacitors and two diodes.
 7. Theultrasound identification sensing tag of claim 5, wherein said modulatorfurther comprises an n-channel metal-oxide-semiconductor field-effecttransistor (MOSFET) having a gate, said gate being controlled by saidoscillation signal.
 8. The ultrasound identification sensing tag ofclaim 5, wherein said modulator is further configured to activelymodulate said input impedance of said integrated circuit chip to cause aperiodic impedance change seen by said piezoelectric transducer, saidperiodic impedance change being detectable by said piezoelectrictransducer.
 9. The ultrasound identification sensing tag of claim 8,wherein said periodic impedance change detectable by said piezoelectrictransducer causes a periodic change in an acoustic impedance mismatchbetween said ultrasound identification sensing tag and one or moresurrounding tissues, said acoustic impedance mismatch creating an echosignal transmitted from said piezoelectric transducer and returned backto said commercial ultrasound imaging equipment; wherein said echosignal forms a brightness-mode (B-mode) image in said commercialultrasound imaging equipment and causes a periodic brightness change ofsaid ultrasound identification sensing tag in said brightness-mode(B-mode) image, indicating said ultrasound identification sensing tag isfunctional.
 10. The ultrasound identification sensing tag of claim 1,wherein said ultrasound identification sensing tag further comprisessensors for receiving one or more physiological signals from abiological tissue to enable applications including at least one ofreal-time pH sensing, drug delivery and controlled release, chemical andbiological activity detection, and digestion monitoring.
 11. Theultrasound identification sensing tag of claim 1, wherein saidintegrated circuit chip further comprises a modulator coupled to arelaxation oscillator, wherein said modulator is configured to receivean oscillation signal from said relaxation oscillator and activelymodulate said input impedance of said integrated circuit chip to cause aperiodic impedance change seen by said piezoelectric transducer, saidperiodic impedance change being detectable by said piezoelectrictransducer, wherein said periodic impedance change detectable by saidpiezoelectric transducer causes a periodic change in an acousticimpedance mismatch between said ultrasound identification sensing tagand one or more surrounding tissues, said acoustic impedance mismatchcreating an echo signal transmitted from said piezoelectric transducerand returned back to said commercial ultrasound imaging equipment, andwherein said echo signal forms a brightness-mode (B-mode) image in saidcommercial ultrasound imaging equipment and causes a periodic brightnesschange of said ultrasound identification sensing tag in saidbrightness-mode (B-mode) image, indicating said ultrasoundidentification sensing tag is functional.
 12. The ultrasoundidentification sensing tag of claim 1, wherein said integrated circuitchip has a thickness of approximately 20 μm.
 13. The ultrasoundidentification sensing tag of claim 1, wherein said piezoelectrictransducer has a length of approximately 150 μm, a width ofapproximately 95 μm and a thickness of approximately 28 μm.
 14. Theultrasound identification sensing tag of claim 1, wherein saidultrasound identification sensing tag has a length of approximately 150μm, a width of approximately 150 μm and a thickness of approximately 48μm.
 15. The ultrasound identification sensing tag of claim 7, whereinsaid n-channel MOSFET is a single n-channel MOSFET.
 16. The ultrasoundidentification sensing tag of claim 1, wherein said piezoelectrictransducer is integrated on a surface of said integrated circuit chip.17. An ultrasound identification sensing tag powered by ultrasoundsignals from commercial ultrasound imaging equipment, comprising: apiezoelectric transducer configured to convert mechanical energy of saidultrasound signals into alternating-current (AC) electrical energy; andan integrated circuit chip, electrically coupled to said piezoelectrictransducer, comprising: (i) circuitry configured to at least receivesaid electrical energy for power and to actively modulate an inputimpedance of the integrated circuit chip; (ii) a charge pump, coupled toreceive said AC electrical energy from said piezoelectric transducer,configured to produce a direct-current (DC) output voltage; (iii) arelaxation oscillator, coupled to said charge pump, configured toreceive said DC output voltage from said charge pump and generate anoscillation signal; and (iv) a modulator, coupled to said relaxationoscillator, configured to (i) receive said oscillation signal from saidrelaxation oscillator and modulate said input impedance of saidintegrated circuit chip and (ii) actively modulate said input impedanceof said integrated circuit chip to cause a periodic impedance changeseen by said piezoelectric transducer, said periodic impedance changebeing detectable by said piezoelectric transducer, wherein saidintegrated circuit chip and said piezoelectric transducer areintegrated, and wherein said periodic impedance change detectable bysaid piezoelectric transducer causes a periodic change in an acousticimpedance mismatch between said ultrasound identification sensing tagand one or more surrounding tissues, said acoustic impedance mismatchcreating an echo signal transmitted from said piezoelectric transducerand returned back to said commercial ultrasound imaging equipment;wherein said echo signal forms a brightness-mode (B-mode) image in saidcommercial ultrasound imaging equipment and causes a periodic brightnesschange of said ultrasound identification sensing tag in saidbrightness-mode (B-mode) image, indicating said ultrasoundidentification sensing tag is functional.
 18. An ultrasoundidentification sensing tag powered by ultrasound signals from commercialultrasound imaging equipment, comprising: a piezoelectric transducerconfigured to convert mechanical energy of said ultrasound signals intoalternating-current (AC) electrical energy; and an integrated circuitchip, electrically coupled to said piezoelectric transducer, comprising:(i) circuitry configured to at least receive said electrical energy forpower and to actively modulate an input impedance of the integratedcircuit chip, and (ii) a modulator coupled to a relaxation oscillator,wherein said modulator is configured to receive an oscillation signalfrom said relaxation oscillator and actively modulate said inputimpedance of said integrated circuit chip to cause a periodic impedancechange seen by said piezoelectric transducer, said periodic impedancechange being detectable by said piezoelectric transducer, wherein saidperiodic impedance change detectable by said piezoelectric transducercauses a periodic change in an acoustic impedance mismatch between saidultrasound identification sensing tag and one or more surroundingtissues, said acoustic impedance mismatch creating an echo signaltransmitted from said piezoelectric transducer and returned back to saidcommercial ultrasound imaging equipment, wherein said echo signal formsa brightness-mode (B-mode) image in said commercial ultrasound imagingequipment and causes a periodic brightness change of said ultrasoundidentification sensing tag in said brightness-mode (B-mode) image,indicating said ultrasound identification sensing tag is functional, andwherein said integrated circuit chip and said piezoelectric transducerare integrated.